Static semiconductor memory device capable of enhancing access speed

ABSTRACT

In a static memory cell including first and second drive MOS transistors, first and second MOS transfer transistors and first and second load elements, the drain of the first drive MOS transistor and the source of the first transfer MOS transistor are formed by a first impurity region in a semiconductor substrate, and the drain of the second drive MOS transistor and the source of the second transfer MOS transistor are formed by a second impurity region in the semiconductor substrate. Also, a first metal silicide layer is formed on the first impurity region and the gate of the second drive MOS transistor, and a second metal silicide layer is formed on the second impurity region and the gate of the the drive MOS transistor. Further, the first and second load elements are formed on the first and second metal silicide layers, respectively.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 09/032,488 filed Feb. 27, 1998, now U.S. Pat. No. 6,178,110.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a static semiconductor memory device, and more particularly, to the improvement of a static random access memory (SRAM) cell.

2. Description of the Related Art

A prior art SRAM cell is constructed by a flip-flop formed by cross-coupled first and second inverters and transfer transistors connected between first and second nodes of the flip-flop and data lines. That is, the first inverter is formed by a first load resistor between a power supply line and the first node and a drive MOS transistor between the first node and a ground line. Similarly, the second inverter is formed by a second load resistor element between the power supply line and the second node and a second drive MOS transistor between the second node and the ground line. In this SRAM cell, in order to increase the access speed, a salidation technology has been adopted. For example, the gate electrodes are constructed by a double configuration made of polycrystalline silicon and metal silicide. This will be explained later in detail.

In the above-described prior art SRAM cell however, since the load resistors are formed on the same plane as the gates of the drive transistors and the transfer transistors, the area of the SRAM cell is increased, which is disadvantageous in terms of integration.

If the load resistor is formed over the gates of the drive transistors, a parasitic resistance between the gate of the drive transistor and the source of the transfer transistor at the node is increased, thus remarkably decreasing the access speed of the SRAM cell. This also will be explained later in detail.

SUMMARY OF THE INVENTION

It is an object of the invention to enhance the access speed of an SRAM cell as well as improve the integration thereof.

According to the present invention, in a static memory cell including first and second drive MOS transistors, first and second MOS transfer transistors and first and second load elements, the drain of the first drive MOS transistor and the source of the first transfer MOS transistor are formed by a first impurity region in a semiconductor substrate, and the drain of the second drive MOS transistor and the source of the second transfer MOS transistor are formed by a second impurity region in the semiconductor substrate. Also, a first metal silicide layer is formed on the first impurity region and the gate of the second drive MOS transistor, and a second metal silicide layer is formed on the second impurity region and the gate of the drive MOS transistor. Further, the first and second load elements are formed on the first and second metal silicide layers, respectively.

Thus, since the load elements are formed on a different plane from that of the transfer transistors and the drive transistors, the size of the SRAM cell can be reduced. Also, since the metal silicide layers are formed at the contact nodes, the parasitic resistance thereof can be reduced, thus increasing the access speed of the SRAM cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from the description as set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein:

FIG. 1 is an equivalent circuit diagram illustrating a first prior art SRAM cell;

FIG. 2 is a plan view of the SRAM cell of FIG. 1;

FIGS. 3A through 3H are cross-sectional views of the SRAM cell of FIG. 2;

FIG. 4 is a cross-sectional view illustrating a second prior art SRAM cell;

FIG. 5 is an equivalent circuit diagram illustrating a first embodiment of the SRAM cell according to the present invention;

FIGS. 6A and 6B are plan views of the SRAM cell of FIG. 5;

FIGS. 7A through 7J are cross-sectional views of the SRAM cell of FIGS. 6A and 6B;

FIG. 8 is an equivalent circuit diagram illustrating a second embodiment of the SRAM cell according to the present invention;

FIGS. 9A and 9B are plan views of the SRAM cell of FIG. 8; and

FIGS. 10A through 10M are cross-sectional views of the SRAM cell of FIGS. 9A and 9B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the preferred embodiments, a prior art SRAM cell will be explained with reference to FIGS., 1, 2, 3A through 3H and 4.

In FIG. 1, which is an equivalent circuit diagram illustrating the prior art SRAM cell, one SRAM is provided at each intersection between a word line WL and two complementary bit lines BL₁ and BL₂. The memory cell is constructed by-a flip-flop formed by two cross-coupled inverters and transfer N-channel MOS transistors Q_(t1) and Q_(t2) connected between nodes N₁ of the flip-flop and bit lines BL₁ and BL₂. The transfer transistors Q_(t1) and Q_(t2) are controlled by the voltage at the word line WL.

Each of the inverters includes a load resistor R₁ (R₂) and a drive N-channel MOS transistor Q_(d1) (Q_(d2)) between a power supply line V_(cc) and a ground line GND. The node N₁ is connected to a gate of the drive transistor Q_(d2), so that the drive transistor Q_(d2) is driven by the voltage at the node N₁. Also, the node N₂ is connected to a gate of the drive transistor Q_(d1), so that the drive transistor Q_(d1) is driven by the voltage at the node N₂.

FIG. 2 is a plan view of the SRAM cell of FIG. 1, and FIGS. 3A through 3H are cross-sectional views taken along the line III—III of FIG. 2. Note that the SRAM cell as illustrated in FIG. 2 is asymmetrical.

The structure of the SRAM cell of FIG. 1 is explained next with reference to FIGS. 3A through 3H which show manufacturing steps thereof, as well as FIG. 2.

First, referring to FIG. 3A, a P-type monocrystalline silicon substrate 101 is thermally oxidized by a local oxidation of silicon (LOCOS) process to form an about 600 nm thick field silicon oxide layer 102. As a result, an active area AR (see FIG. 2) is surrounded by the field silicon oxide layer 102. Then, an about 20 nm thick gate silicon oxide layer 103 is grown by thermally oxidizing the substrate 101.

Next, referring to FIG. 3B, an about 300 nm thick undoped polycrystalline silicon layer 104 is deposited on the entire surface by a chemical vapor deposition (CVD) process. Then, a photoresist pattern 105 is formed, and about 10¹⁵ phosphorus ions per cm² are implanted at an energy of about 30 keV. As a result, the resistance values of portions of the polycrystalline silicon layer 104 beneath the photoresist pattern 105 corresponding to the load resistors R₁ and R₂ remain high, while the resistance values of the other portions becomes low. Then, the photoresist pattern 105 is removed.

Next, referring to FIG. 3C, the polycrystalline silicon layer 104 is patterned by a photolithography process, so that the word line WL (i.e., the gates of the transfer transistors Q_(t1) and Q_(t2)), the load resistors R₁ and R₂ and the gates of the drive transistors Q_(d1) and Q_(d2) are formed (see FIG. 2). Then, about 2×10¹³ phosphorus ions per cm² at an energy of about 10 keV are implanted into the substrate 101 using the polycrystalline silicon layer 104 and the field silicon oxide layer 102 as a mask, so that N⁻-type impurity diffusion regions 106 of a lightly-doped drain (LDD) configuration are formed.

Next, referring to FIG. 3D, an about 150 nm thick silicon oxide layer 107 is deposited on the entire surface by a CVD process. Then, a photoresist pattern 108 is formed.

Next, referring to FIG. 3E, the silicon oxide layer 107 is etched back by an anisotropic process to form a sidewall silicon oxide layer 107 a. In this case, the silicon oxide layer 107 is left as a silicon oxide layer 107 beneath the photoresist pattern 108. Then, the photoresist pattern 108 is removed.

Next, referring to FIG. 3F, about 4×10¹⁵ arsenic ions per cm² are implanted into the substrate 101 by using the sidewall silicon oxide layer 107 a as a mask to form N⁺-type impurity diffusion regions 109 of the LDD configuration (see FIG. 2). In this case, the load resistor R₁ (R₂) of the polycrystalline silicon layer 104 is not subjected to the arsenic ions, so that the resistance value of the load resistor R₁ (R₂) remains high.

Next, referring to FIG. 3G, an about 50 to 60 nm thick titanium layer is deposited on the entire surface by a sputtering process. Then, a heating operation is performed upon the titanium layer at a temperature of about 650° C. for about 30 seconds, so that the titanium layer is reacted with the polycrystalline silicon layer 104. Then, the unreacted portion of the titanium layer is removed by an alkali solution such as ammonia water/hydrogen peroxide water mixture (APM). As a result, a titanium silicide layer 110 is formed only on the polycrystalline silicon layer 104. Then, a heating operation is carried out at a temperature of about 760° C. for about 20 seconds to decrease the resistance of the titanium layer 110. Note that, if such a heating operation is carried out at a too high temperature for a longer time period than about 20 seconds, the titanium layer 110 may agglomerate, so that the resistance thereof increases.

Finally, referring to FIG. 3H, a silicon oxide layer 111 is deposited on the entire surface by a CVD process. Then, contact holes CONT₁, CONT₂, CONT₃ and CONT₄ are perforated in the silicon oxide layer 111 (see FIG. 2). Then, an aluminum layer is deposited and patterned to form an electrode pattern 112. Note that the contact hole CONT₁ is used for the node N₁, the contact hole CONT₂ is used for the node N₂, the contact hole CONT₃ is used for connecting the gate (the node N₁) of the drive transistor Q_(d2) to the source of the transfer transistor Q_(t1), and the contact hole CONT₄ is used for a power supply line Vcc (not shown).

Thus, the SRAM cell of FIG. 2 is completed.

In the above-described SRAM cell as illustrated in FIGS. 2 and 3A through 3H, however, since the load resistors R₁ and R₂ are formed on the same plane as the gates of the transistors Q_(t1), Q_(t2), Q_(d1) and Q_(d2), the area of the SRAM cell is increased, which is disadvantageous in terms of integration.

If the load resistor R₁ (R₂) is formed over the gates of the transistors Q_(t1), Q_(t2), Q_(d1) and Q_(d2), a low-resistance polycrystalline silicon layer 112′ and a high-resistance polycrystalline silicon layer 112′ (R₁) as illustrated in FIG. 4 can be provided instead of the aluminum electrode pattern 112 of FIG. 3H. That is, in FIG. 4, the polycrystalline silicon layer has a high concentration of impurities, and the polycrystalline silicon layer 112′ (R₁) has a low concentration of impurities. In this case, however, when a heating operation for the titanium layer 110 is carried out, the resistance of the polycrystalline silicon layer 112′ is increased in spite of including a high concentration of impurities. As a result, a parasitic resistance between the gate of the drive transistor Q_(d2) (Q_(d1)) and the source of the transfer transistor Q_(t1) (Q_(t2)) at the node N₁ (N₂) is increased, thus remarkably decreasing the access speed of the SRAM cell of FIG. 1.

In FIG. 5, which is an equivalent circuit diagram illustrating a first embodiment of the SRAM cell according to the present invention, two word lines WL₁ and WL₂ are provided instead of the word line WL of FIG. 1. That is, the transfer transistor Q_(t1) is controlled by the voltage at the word line WL₁, and the transfer transistor Q_(t2) is controlled by the voltage at the word line WL₂. In this case, note that the voltage at the word line WL₁ is the same as the voltage at the word line WL₂.

FIGS. 6A and 6B are plan views of the SRAM cell of FIG. 5, and FIGS. 7A through 7J are cross-sectional views taken along the line VII—VII of FIGS. 6A and 6B. Note that the SRAM cell as illustrated in FIGS. 6A and 6B is symmetrical.

The structure of the SRAM cell of FIG. 5 is explained next with reference to FIGS. 7A through 7J which a show manufacturing steps thereof, as well as FIGS. 6A and 6B.

First, referring to FIG. 7A, a P-type monocrystalline silicon substrate 201 is thermally oxidized by a LOCOS process to form an about 400 nm thick field silicon oxide layer 202. As a result, an active area AR (see FIG. 6A) is surrounded by the field silicon oxide layer 202. Then, an about 10 nm thick gate silicon oxide layer 203 is grown by thermally oxidizing the substrate 201.

Next, referring to FIG. 7B, an about 300 nm thick N-type polycrystalline silicon layer 204 is deposited on the entire surface by a low pressure chemical vapor deposition (LPCVD) process. Then, the polycrystalline silicon layer 204 is patterned by a photolithography process and an anisotropic dry etching process, so that the word lines WL₁ and WL₂ (i.e., the gates of the transfer transistors Q_(t1) and Q_(t2)) and the gates of the drive transistors Q_(d1) and Q_(d2) are formed (see FIG. 6A). Then, about 2×10¹³ phosphorus ions per cm² at an energy of about 10 keV are implanted into the substrate 201 using the polycrystalline silicon layer 204 and the field silicon oxide layer 202 as a mask, so that N⁻-type impurity diffusion regions 205 of an LDD configuration are formed.

Next, referring to FIG. 7C, an about 100 nm thick silicon oxide layer is deposited on the entire surface by an LPCVD process. Then, the silicon oxide layer is etched back by a photolithography process and an anisotropic process to form a sidewall silicon oxide layer 206. Then, about 5×10¹⁵ arsenic ions per cm² are implanted into the substrate 201 by using the sidewall silicon oxide layer 206 as a mask to form N⁺-type impurity diffusion regions 207 of the LDD configuration which have an impurity concentration of about 10²⁰ to 10²¹ per cm³ (see FIG. 6A).

Next, referring to FIG. 7D, an about 50 to 60 nm thick titanium layer is deposited on the entire surface by a sputtering process. Then, a heating operation is performed upon the titanium layer at a temperature of lower than 700° C. such as about 650° C. for about 30 seconds, so that the titanium layer is reacted with the polycrystalline silicon layer 204. Then, the unreacted portion of the titanium layer is removed by an alkali solution such as APM. As a result, a titanium silicide layer 208 is formed only on the polycrystalline silicon layer 204. Then, a heating operation is carried out at a temperature of higher than 800° C. for about 20 seconds to decrease the resistance of the titanium layer 208.

Next, referring to FIG. 7E, an insulating layer 209 is formed on the entire surface. That is, first, an about 100 nm thick silicon oxide layer is deposited by an atmospheric pressure chemical vapor deposition (APCVD) process. Then, an about 600 nm thick boron-included phosphorus silicated glass (BPSG) layer is deposited on the silicon oxide layer by an APCVD process. Then, after a heating operation at about 800° C. is carried out, a chemical mechanical polishing (CMP) process is performed upon the BPSG layer, to flatten it. Then, an about 100 nm thick silicon oxide layer is deposited on the flattened BPSG layer by an APCVD process, thus completing the insulating layer 209.

Next, referring to FIG. 7F, contact holes CONT₁ and CONT₂ are perforated in the insulating layer 209 (see FIG. 6A). In this case, in the contact holes CONT₁ and CONT₂, the gate silicon oxide layer 203 and the sidewall silicon oxide layer 206 are completely etched. However, note that the titanium silicide layer 208 within the contact holes CONT₁ and CONT₂ can be left by using a high etching ratio etching process of silicon oxide to titanium silicide.

Next, referring to FIG. 7G, an about 50 to 60 nm thick titanium layer is deposited on the entire surface by a sputtering process. Then, a heating operation is performed upon the titanium layer at a temperature of lower than 700° C. such as about 650° C. for about 30 seconds, so that the titanium layer is reacted with the N⁺-type impurity region 207 and the polycrystalline silicon layer 204 within the contact holes CONT₁ and CONT². Then, the unreacted portion of the titanium layer is removed by an alkali solution such as SPM. As a result, a titanium silicide layer 210 is formed only within the contact holes CONT¹ and CONT₂. Then, a heating operation is carried out at a temperature of higher than 800° C. for about 20 seconds to decrease the resistance of the titanium layer 210.

Next, referring to FIG. 7H, an about 50 to 100 nm thick polycrystalline silicon (or semi-insulating polycrystalline silicon (SIPOS)) layer is deposited on the entire surface by an LPCVD process. In this case, if the device is in an ammonia plasma atmosphere, nitride is grown on the titanium silicide layer 210, thus preventing active silicon and titanium silicide from reacting with each other when the polycrystalline silicon (or SIPOS) layer is grown. Then, after about 1×10¹³ N-type impurity ions per cm² are implanted into the polycrystalline silicon (or SIPOS) layer, the latter is patterned by a photolithography process and an anisotropic dry etching process, to form a low impurity concentration polycrystalline silicon layer 211.

Next, referring to FIG. 7I, a photoresist pattern 212 is formed. Then, about 1×10¹⁵ phosphorus (or arsenic) ions are implanted into the low impurity concentration polycrystalline silicon layer 211. As a result, although the resistance of the polycrystalline silicon layer 211 beneath the photoresist pattern 212 remain high, the other portion of the polycrystalline silicon layer 211 becomes a polycrystalline silicon layer 211′ having a low resistance (see FIG. 6B). Then, the photoresist 212 is removed.

Finally, referring to FIG. 7J, an insulating layer 212 is formed on the entire surface. That is, first, an about 100 nm thick silicon oxide layer is deposited by an APCVD process. Then, an about 600 nm thick BPSG layer is deposited on the silicon oxide layer by an APCVD process. Then, after a heating operation at about 800° C. is carried out, a CMP process is performed upon the BPSG layer, to flatten it.

Then, contact holes CONT₃ and CONT₄ are perforated in the insulating layer 212 (see FIGS. 6A and 6B). Then, a conductive layer 213 made of aluminum or the like is formed on the insulating layer 211 and is patterned. As a result, the conductive layer 213 is connected as the ground line GND via the contact holes CONT₃ and CONT₄ to the sources of the drive transistors Q_(d1) and Q_(d2).

Further, an insulating layer 214 is formed on the entire surface. Then, contact holes CONT₅ and CONT₆ are perforated in the insulating layer 214 (see FIGS. 6A and 6B). Then, metal plugs 215 are buried in the contact holes CONT₅ and CONT₆. Then, a conductive layer 216 made of aluminum or the like is formed on the insulating layer 214 and is patterned. As a result, the conductive layer 214 is connected as the data lines DL₁ and DL₂ via the contact holes CONT₅ and CONT₆ to the drains of the transfer transistors Q_(t1) and Q_(t2).

Thus, the SRAM cell of FIGS. 6A and 6B is completed.

In the SRAM cell as illustrated in FIGS. 5, 6A and 6B and 7A through 7J, since the load resistors R₁ and R₂ are formed on a different plane from that of the gates of the transistors Q_(t1), Q_(t2), Q_(d1) and Q_(d2), the area of the SRAM cell is decreased, which is advantageous in terms of integration. Also, since the titanium silicide layer 210 is provided between the gate of the drive transistor Q_(d2) (Q_(d1)) and the source of the transfer transistor Q_(t1) (Q_(t2)) at the nodes N₁ (N₂), the parasitic resistance at the node N₁ (N₂) is remarkably reduced, thus increasing the access speed of the SRAM cell of FIG. 5.

In FIG. 8, which is an equivalent circuit diagram illustrating a second embodiment of the SRAM cell according to the present invention, P-channel thin film transistors (TFTs) Q_(p1) and Q_(p2) are provided instead of the load resistors R₁ and R₂, respectively, of FIG. 5.

FIGS. 9A and 9B are plan views of the SRAM cell of FIG. 8, and FIGS. 10A through 10M are cross-sectional views taken along the line X—X of FIGS. 9A and 9B. Note that the SRAM cell as illustrated in FIGS. 9A and 9B is symmetrical.

The structure of the SRAM cell of FIG. 8 is explained next with reference to FIGS. 10A through 10J which show manufacturing steps thereof, as well as FIGS. 9A and 9B.

First, referring to FIG. 10A, in the same way as in FIG. 7A, a P-type monocrystalline silicon substrate 301 is thermally oxidized by a LOCOS process to form an about 400 nm thick field silicon oxide layer 302. As a result, an active area AR (see FIG. 9A) is surrounded by the field silicon oxide layer 302. Then, an about 10 nm thick gate silicon oxide layer 303 is grown by thermally oxidizing the substrate 301.

Next, referring to FIG. 10B, in the same way as in FIG. 7B, an about 300 nm thick N-type polycrystalline silicon layer 304 is deposited on the entire surface by a LPCVD process. Then, the polycrystalline silicon layer 304 is patterned by a photolithography process and an anisotropic dry etching process, so that the word lines WL₁ and WL₂ (i.e., the gates of the transfer transistors Q_(t1) and Q_(t2)) and the gates of the drive transistors Q_(d1) and Q_(d2) are formed (see FIG. 9A). Then, about 2×10¹³ phosphorus ions per cm² at an energy of about 10 keV are implanted into the substrate 301 using the polycrystalline silicon layer 304 and the field silicon oxide layer 302 as a mask, so that N⁻-type impurity diffusion regions 305 of an LDD configuration are fomed.

Next, referring to FIG. 10C, in the same way as in FIG. 7C, an about 100 nm thick silicon oxide layer is deposited on the entire surface by an LPCVD process. Then, the silicon oxide layer is etched back by a photolithography process and an anisotropic process to form a sidewall silicon oxide layer 306. Then, about 5×10¹⁵ arsenic ions per cm² are implanted into the substrate 301 by using the sidewall silicon oxide layer 306 as a mask to form N⁺-type impurity diffusion regions 307 of the LDD configuration which have an impurity concentration of about 10²⁰ to 10²¹ per cm³ (see FIG. 9A).

Next, referring to FIG. 10D, in the same way as in FIG. 7D, an about 50 to 60 nm thick titanium layer is deposited on the entire surface by a sputtering process. Then, a heating operation is performed upon the titanium layer at a temperature of lower than 700° C. such as about 650° C. for about 30 seconds, so that the titanium layer is reacted with the polycrystalline silicon layer 304. Then, the unreacted portion of the titanium layer is removed by an alkali solution such as SPM. As a result, a titanium silicide layer 308 is formed only on the polycrystalline silicon layer 304. Then, a heating operation is carried out at a temperature of higher than 800° C. for about 20 seconds to decrease the resistance of the titanium silicide layer 308.

Next, referring to FIG. 10E, in the same way as in FIG. 7E, an insulating layer 309 is formed on the entire surface. That is, first, an about 100 nm thick silicon oxide layer is deposited by an APCVD process. Then, an about 600 nm thick BPSG layer is deposited on the silicon oxide layer by an APCVD process. Then, after a heating operation at about 800° C. is carried out, a CMP process is performed upon the BPSG layer, to flatten it. Then, an about 100 nm thick silicon oxide layer is deposited on the flattened BPSG layer by an APCVD process, thus completing the insulating layer 309.

Next, referring to FIG. 10F, in the same way as in FIG. 7F, contact holes CONT₁ and CONT₂ are perforated in the insulating layer 309 (see FIG. 9A). In this case, in the contact holes CONT₁ and CONT₂, the gate silicon oxide layer 303 and the sidewall silicon oxide layer 306 are completely etched. However, note that the titanium silicide layer 308 within the contact holes CONT₁ and CONT₂ can be left by using a high etching ratio etching process of silicon oxide to titanium silicide.

Next, referring to FIG. 10G, in the same way as in FIG. 7G, an about 50 to 60 nm thick titanium layer is deposited on the entire surface by a sputtering process. Then, a heating operation is performed upon the titanium layer at a temperature of lower than 700° C. such as about 650° C. for about 30 seconds, so that the titanium layer is reacted with the N⁺-type impurity region 307 and the polycrystalline silicon layer 304 within the contact holes CONT₁ and CONT₂. Then, the unreacted portion of the titanium layer is removed by an alkali solution such as SPM. As a result, a titanium silicide layer 310 is formed only within the contact holes CONT₁ and CONT₂. Then, a heating operation is carried out at a temperature of higher than 800° C. for about 20 seconds to decrease the resistance of the titanium layer 310.

Next, referring to FIG. 10H, an about 50 to 100 thick polycrystalline silicon layer 311 is deposited on the entire surface by an LPCVD process. In this case, if the device is in a ammonia plasma atmosphere, nitride is grown on the titanium silicide layer 310, thus preventing active silicon and titanium silicide from reacting with each other when the polycrystalline silicon layer 311 is grown. Then, after about 1×10¹³ to 1×10¹⁵ N-type impurity ions per cm² are implanted into the polycrystalline silicon layer 311, the latter is patterned by a photolithography process and an anisotropic dry etching process, to form the gates of the TFTs Q_(p1) and Q_(p2).

Next, referring to FIG. 10I, an about 5 to 20 nm thick gate silicon oxide layer 313 for the TFTs Q_(p1) and Q_(p2) is deposited on the entire surface by an LPCVD process. Then, contact holes CONT₃ and CONT₄ (see FIG. 9B) are perforated in the gate silicon oxide layer 313 by an anisotropic etching process. Thus, the gate of the TFTs Q_(p1) and Q_(d2) are exposed by the contact holes CONT₃ and CONT₄.

Next, referring to FIG. 10J, an amorphous silicon layer is grown on the entire surface, and a heating operation at about 600° C. is performed upon the amorphous silicon layer. As a result, the amorphous silicon layer is changed into a polycrystalline silicon layer 314. After about 1×10¹³ to 10¹³ N-type impurities per cm² are implanted into the polycrystalline silicon layer 314, the polycrystalline silicon layer is patterned by an anisotropic dry etching process.

Next, referring to FIG. 10K, a photoresist patte 315 is formed. Then, about 1×10¹⁴ to 10¹⁵ boron ions (B+ or BF₂ ⁺) per cm2 are implanted into the polycrystal line silicon layer 114 by using the photoresist pattern 115 as a mask. Then, the photoresist pattern 315 is removed. As a result, as illustrated in FIG. 10L, the polycrystalline silicon layer 314 is divided into a channel region 314 (C), a source region 314 (S) and a drain region 314 (D) of each of the TFTs Q_(p1) and Q_(p2) (see FIG. 9B).

Finally, referring to FIG. 10M, in the same way as in FIG. 7J, an insulating layer 316 is formed on the entire surface. That is, first, an about 100 nm thick silicon oxide layer is deposited by an APCVD process. Then, an about 600 nm thick BPSG layer is deposited on the silicon oxide layer by an APCVD process. Then, after a heating operation at about 800° C. is carried out, a CMP process is performed upon the BPSG layer, to flatten it.

Then, contact holes CONT₅ and CONT₆ are perforated in the insulating layer 316 (see FIGS. 9A and 9B). Then, a conductive layer 317 made of aluminum or the like is formed on the insulating layer 316 and is patterned. As a result, the conductive layer 317 is connected as the ground line GND via the contact holes CONT₅ and CONT₆ to the sources of the drive transistors Q_(d1) and Q_(d2).

Further,.an insulating layer 318 is formed on the entire surface. Then, contact holes CONT₇ and CONT₈are perforated in the insulating layer 318 (see FIGS. 9A and 9B). Then, metal plugs 319 are buried in the contact holes CONT₇ and CONT₈. Then, a conductive layer 320 made of aluminum or the like is formed on the insulating layer 318 and is patterned. As a result, the conductive layer 320 is connected as the data lines DL₁ and DL₂ via the contact holes CONT₇ and CONT₈ to the drains of the transfer transistors Q_(t1) and Q_(t2).

Thus, the SRAM cell of FIGS. 9A and 9B is completed.

In the SRAM cell as illustrated in FIGS. 8, 9A and 9B and 10A through 10K, since the TFTs are formed on a different plane from that of the gates of the transistors Q_(t1) , Q_(t2), Q_(d1) and Q_(d2) , the area of the SRAM cell is decreased, which is advantageous in terms of integration. Also, since the titanium silicide layer 310 is provided between the gate of the drive transistor Q_(d2) (Q_(d1)) and the source of the transfer transistor Q_(t1) (Q_(t2)) at the nodes N₁ (N₂), the parasitic resistance at the node N₁ (N₂) is remarkably reduced, thus increasing the access speed of the SRAM cell of FIG. 8.

In the above-described second embodiment, the gates of the TETs are beneath the channel regions thereof, however, the present invention can be applied to an SRAM cell where the gates of TFTs are above the channel regions thereof.

As explained hereinabove, according to the present invention, since the load resistors or the TFTs are formed on a different plane from that of the transfer transistors and the drive transistors, the size of the SRAM cell can be reduced. For example, the area of the SRAM cell can be reduced by about 30 percent as compared with the prior art SRAM cell of FIGS. 1, 2 and 3A through 3H. Also, since the metal silicide layers are formed at the contact nodes, the parasitic resistance thereof can be reduced, thus increasing the access speed of the SRAM cell. For example, the parasitic resistance can be reduced by a factor of hundreds as compared with the prior art SRAM cell of FIG. 4. 

What is claimed is:
 1. A method of manufacturing a semiconductor device, comprising: forming a first transistor on a semiconductor substrate; forming an insulating layer on the first transistor; forming a contact hole in the insulating layers, said contact hole exposing a drain of said first drive transistor; depositing metal silicide layer in the contact hole; and forming a load element on the insulating layer, wherein the load element is coupled to the first transistor at the contact hole.
 2. A method, according to claim 1, further comprising: forming a second transistor on the semiconductor substrate, the second transistor being coupled to the first transistor at the contact hole, whereby parasitic resistance at the contact hole is minimized by the metal silicide.
 3. A method of manufacturing a semiconductor memory device, comprising: forming first and second drive transistors on a semiconductor substrate; forming an insulating layer on the drive transistors; forming first and second contact holes in the insulating layer,said first contact hole exposing first drain of said first drive transistor, said second contact hole exposing second drain of said second drive transistor; depositing metal silicide in the contact holes; and forming first and second load elements on the insulating layer, wherein the first load element is coupled to the first drive transistor at the first contact hole and the second load element is coupled to the second drive transistor at the second contact hole.
 4. A method, according to claim 3, further comprising: forming first and second transfer transistors on the semiconductor substrate, the transfer transistiors being coupled to the drive transistors via the metal silicide in the contact holes, whereby parasitic resistance at the contact nodes is minimized by the metal silicide.
 5. A method, according to claim 3, wherein forming the first and second load elements includes forming polycrystalline resistors.
 6. A method, according to claim 3, wherein forming the first and second load elements includes forming thin film transistors.
 7. A method for manufacturing a static memory cell including first and second drive MOS transistors, first and second MOS transfer transistors and first and second load elements, comprising the steps of: forming a first polycrystalline silicon layer on a semiconductor substrate; patterning said first polycrystalline silicon layer, so that gates of said first and second drive MOS transistors and said first and second transfer MOS transistors are formed; implanting first impurities into said semiconductor substrate using the gates of said first and second drive MOS transistors and said first and second transfer MOS transistors as a mask; forming sidewall insulating layers on sidewalls of the gates of said first and second drive MOS transistors and said first and second transfer MOS transistors, after said first impurities are implanted; implanting second impurities into said semiconductor substrate using the gates of said first and second drive MOS transistors and said first and second transfer MOS transistors and said sidewall insulating layers as a mask, so that sources and drains of said first and second drive MOS transistors and said first and second transfer MOS transistors are formed; forming a first metal silicide layer on the gates of said first and second drive MOS transistors and said first and second transfer MOS transistors after said second impurities are implanted; forming a first insulating layer on an entire surface of said static memory cell after said first metal silicide layer is formed; perforating first and second contact holes in said first insulating layer and said sidewall insulating layers, said first contact hole exposing the drain of said first drive MOS transistor, the source of said first transfer MOS transistor and the gate of said second drive MOS transfer, said second contact hole exposing the drain of said second drive MOS transistor, the source of said second transfer MOS transistor and the gate of said first drive MOS transfer; forming a second metal silicide layer within said first and second contact holes; and forming said first and second load elements on said second metal silicide layer and said first insulating layer.
 8. The method as set forth in claim 7, wherein said first and second load element forming step comprises the steps of: forming a second polycrystalline silicon layer on said second metal silicide layer and said first insulating layer; and; partly implanting third impurities into said second polycrystalline silicon layer.
 9. The method as set forth in claim 7, wherein said first and second load element forming step comprises the steps of: forming a second polycrystalline silicon layer on said second metal silicide layer and said first insulating layer; forming a second insulating layer on said second polycrystalline silicon layer; and forming a third polycrystalline silicon layer, said second and third polycrystalline layers with said second insulating layer forming thin film transistors.
 10. The method as set forth in claim 7, further comprising a step of exposing said static memory cell in an ammonia plasma atmosphere after said second metal silicide layer is formed and before said first and second load elements are formed. 